Replicate the exporter metadata

[saig0]

Description

An exporter can submit metadata. The metadata is stored in the exporter state. The exporter can read the metadata from the state on startup.

In order to handle leader changes gracefully, we need to replicate the exporter metadata. Currently, we already replicate the exporter positions from the leader to the followers. The followers accept the exporter positions and store them in their state. As a result, the exporter can continue from the last position (based on the last update) after a leader change.

We should replicate the exporter metadata next to the exporter position. The followers should write the exporter metadata in their state together with the exporter's position.

This issue is part of #10568.